1. Field of the Invention
The present invention relates to an organic electro luminescence display device, and more particularly to an organic electro luminescence display device using pre-charge, and a driving method thereof.
2. Description of the Related Art
Recently, there have been developed various flat panel display devices of which the weight and size can be reduced, wherein the weight and size is a disadvantage of a cathode ray tube CRT. The flat panel display device includes liquid crystal display (hereinafter, referred to as “LCD”), field emission display (hereinafter, referred to as “FED”), plasma display panel (hereinafter, referred to as “PDP”), and electro-luminescence display (hereinafter, referred to as “EL”).
The PDP has a relatively simple structure and fabricating process, thus the PDP is advantageous in being made into a large screen, but there is a disadvantage in that its light emission efficiency and brightness is low and its power consumption is high.
The LCD has its demand increased as it is mainly used as a display device of a notebook computer. However, the LCD is fabricated by a semiconductor process, thus it is difficult to be made into the large screen. And, because the LCD is not a self-luminous device, there is a disadvantage in that a separate light source is required and power consumption is big due to the light source. Further, the LCD has a disadvantage in that a lot of light loss is caused by optical devices such as a polarizing filter, a prism sheet, a diffusion plate, etc and its viewing angle is narrow.
The EL display device is roughly divided into an inorganic EL display device and an organic EL display device, and has an advantage in that its response speed is fast and its light emission efficiency, brightness and viewing angle are high. The organic EL display device can display a picture in a high brightness of tens of thousands [cd/m2] with a voltage of about 10[V] or so, and is applied to most of the EL display devices which are put to practical use.
A unit device of the organic EL display device, as shown in FIG. 1, forms an anode 2 of transparent conductive material on a glass substrate 1, and a hole injection layer 3, a light emitting layer of organic material and a cathode 5 of metal of which a work function is low, on top thereof. If an electric field is applied between the anode 2 and the cathode 5, holes within the hole injection layer 3 and electrons within the metal respectively move toward the light emitting layer 4 to be combined with each other in the light emitting layer 4. Then, a fluorescent material within the light emitting layer 4 is excited to have a transition made, thereby generating a visible ray. At this moment, the brightness is proportional to a current between the anode 2 and the cathode 5.
The organic EL display device is divided into a passive type and an active type.
FIG. 2 is a circuit diagram equivalently representing a part of an organic EL display device of a passive type, and FIG. 3 is a waveform diagram representing waveforms of a scan signal and a data signal of the passive type organic EL display device.
Referring to FIGS. 2 and 3, the passive type organic EL display device includes a plurality of data lines D1 to Dm and a plurality of scan lines S1 to Sn which cross each other; and organic EL cells OLED respectively formed at the crossing parts between the data lines D1 to Dm and the scan lines S1 to Sn.
The data lines D1 to Dm are connected to the anode of the organic EL device OLED to supply a data current Id to the anode of the organic EL device OLED.
The scan lines S1 to Sn is connected to the cathode of the organic EL cell OLED to supply scan pulses SP1 to SPn synchronized with the data current Id to the cathode of the organic EL cell OLED.
The organic EL cell OLED emits light in proportion to the current flowing between the anode and the cathode during a display period DT when the scan pulses SP1 to SPn are applied.
The organic EL cell OLED of the organic EL display device has a problem that its response speed is low and its brightness is low because a current is charged therein during a response time RT delayed by a capacitance existing in the organic EL cell OLED and a resistance component of the data lines D1 to Dm. In order to compensate the low response speed of the organic EL cell OLED, there is recently proposed a technique that a pre-charge period is provided as a non-display period between display periods DT.
On the other hand, in the organic EL display device, in case of realizing a designated picture, a pre-charge voltage waveform in contradiction to the same pre-charge period is made differently in accordance with locations of the data lines D1 to Dm and the scan lines S1 to Sn, thereby generating a problem such as a horizontal cross talk.
In reference to FIGS. 4 and 5, a description will be made in detail as follows.
Firstly, FIG. 4 represents a still picture has a black picture realized in the middle of the picture and a white picture realized in an area except the black picture.
In this case, even though the data current and the pre-charge current for representing the same brightness are applied between a white picture area (hereinafter, referred to as “first white area A”) horizontally adjacent to and a white picture area (hereinafter, referred to as “second white area B”) vertically adjacent to the area where the black picture is realized, there is generated a drive voltage (current) deviation due to a difference of loading quantity in a horizontal line direction.
In other words, the organic EL cells OLED corresponding to the black picture area in an (N+1)th scan line do not emit light, thus the loading quantity corresponding to non-light-emitting organic EL cells OLED is excluded from the total loading quantity in the (N+1)th scan line. Accordingly, a higher drive voltage is charged in the second white area B than in the first white area A, wherein the second white area B corresponds to the Nth scan line of which the loading quantity is relatively larger than the (N+1)th scan line.
After this, even though the pre-charge current of the same size is applied to the first white area A and the second white area B, the drive voltage deviation between the first white area A and the second white area B is maintained intact.
As a result, as shown in FIG. 5, the amount of the current charged in the data line corresponding to the (N+1)th scan line in contradiction to the same pre-charge period is relatively lower than the amount of the current charged in the data line corresponding to the Nth scan line, thus there is generated a cross talk problem by the brightness difference (or drive voltage deviation) between the first white area A and the second white area B although the same data current and pre-charge current is supplied for expressing the same gray level.